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Ordnance Technology Propellant Components Gun Propulsion Cartridge Actuated Devices Propellant Actuated Devices Rockets and Missile Systems Explosives and Warheads PROPELLANT COMPONENTS The major ingredients of modern military propellants are actually few. They consist of fuels, oxidizers and binders (polymers), and are fairly basic in their chemical nature and structure. The minor ingredients, used to assist or tie together the major ingredients, are more numerous and sometimes more complex. The interactions of these major and minor ingredients, when combined into a practical solid propellant, are especially complex. These interactions can take place at all stages of manufacture, storage, and use. Controlling such interactions makes solid-propellant technology difficult, expensive and, therefore, important to understand. The ingredients themselves represent output of the conventional chemical and explosive industries (although a few, like nitroglycerin, are produced locally); however, the combination of these ingredients into a propellant is still thought of by some as a "black art." Making it into a science is what has justified the expenditure of so many millions of dollars over the past 50 years. We will review briefly the development of modern propellants, touch on the basic ingredients that are used in the manufacture of propellants, and then describe the various types of propellants and their comparative properties. BACKGROUND OF PROPELLANT DEVELOPMENT The history of truly efficient military propellants is a fairly short one in this country, dating from 1900 for guns and from 1942 for rockets. However, the chemistry of propellant ingredients, has a long history. For example, the use of black powder dates back more than 700 years in Europe and probably 1000 years in the Orient. These facts emphasize the point that mastering the really difficult science of propellants comes in understanding the complex interactions that take place in their preparation, storage and use. Progress toward developing truly efficient propellants was necessarily slow until these interactions were defined and controlled; the problem is a continuing one as new ingredients are developed and introduced into improved propellants. Solid rocket propellants have considerably greater complexity than most gun propellants and all liquid propellants. This is so because solid rockets are mechanically much simpler in principle than either guns or liquid-fueled rockets. Therefore, a solid rocket propellant must perform by chemical means many of the jobs that are performed by the hardware in guns and liquid rockets. The success of military solid rockets since 1943 attests to the competence of the interdisciplinary teams assigned to their development, more than to any supposed greater simplicity of the task. Certain other components of missile systems, such as guidance and control, are at a lower stage of maturity only partly because of their great technical difficulty. Because weapons propulsion is viewed by top DOD officials as a very mature technology, less money each year is available for its improvement. This is regrettable because a long cycle is still required to bring a new propellant or propulsion concept from the idea phase to full production. Even such a simple device as the 2.75-inch folding-fin aircraft rocket required a five-year cycle of development (1949 to 1954) largely because of propellant problems. Six to eight years is a more realistic time scale for modern rocket propulsion systems, although with plenty of money, technical talent, and highest priority, complete weapon systems, have been developed in a somewhat shorter time. Such a wealth of resources cannot be made available, obviously, for every military development. Thus, plans must be made for an extended cycle of development for any new weapon propulsion system and already established technology must be used as much as possible. The environmental performance requirements for a new weapon system must be defined and limited as much as possible before embarking on the program. PROPELLANT TYPES Nitrocellulose deserves special attention because it has served as the major ingredient of military propellants since about 1900 in the U.S. and a few years longer in Europe. Experiments leading to the development of nitrocellulose began in Europe prior to 1840, but the German, Schoenbein, gets the most credit for developing a reasonably practical process by about 1845. The Englishman, Frederic Abel, a Frenchman named Vieille, and Sweden's Alfred Nobel are credited with later discoveries which made the use of nitrocellulose fully practical by about 1890. This half century span of research and development from 1840 to 1890 is indicative of the many problems encountered. Indian Head's own Dr. George W. Patterson, was largely responsible in the U.S. for developing fully practical, colloided or gelatinized nitrocellulose propellants for guns, basing his work primarily on the discoveries of Vieille with ether and alcohol solvents. Practical gun propellants, based on uniform mixture of nitrocellulose and nitroglycerin (double- base), a actually older than Vieille's and Patterson's single-base powders. Alfred Nobel deserves much of the credit for such double-base propellants because of his amazing discovery that these two notoriously treacherous materials could be combined even without solvents into a safe substance, which he called Ballistite. The Nobel process of combining them was neither obvious nor very safe for many years; but once intimately combined into a thoroughly colloided or gelatinized mass, nitrocellulose and nitroglycerin became a remarkably tractable, homogeneous material. By far the greatest part of Indian Head's present production of propellants is based on this invention of Alfred Nobel, with additional newer ones. LIQUID PROPELLANTS Although of lesser recent interest militarily compared to solid propellants, liquid propellants were chosen over solids by the U.S. rocket pioneer, Robert Goddard, in his experiments between the two World Wars. Dr. Goddard's explanation was that no suitable solid propellants were available. Currently, solid propellants predominate over liquids in military weapons because of their greater storability and volumetric efficiency. Solid propellants also offer lower life cycle costs and system simplicity. Conversely, liquid propellants, principally oxygen and kerosene-like fuels, have tended to dominate the space propulsion scene until recently. For military applications liquid oxygen is considered unacceptable. Red fuming nitric acid or nitrogen tetroxide tends to be the oxidizer of choice where a military rocket can benefit from a liquid propellant design. Fuels are mixed organic amines or hydrazine and its various alkyl derivatives, such as monomethyl and dimethyl hydrazine (UDMH). Hybrid systems, generally based on nitric acid oxidizer and a low oxidizer-content, rubber-fuel grain, are of some interest, but have not yet reached full operational status. DOUBLE-BASE PROPELLANTS Although Ballistite for guns can (and in Italy for some years did) consist of nothing more than a 50:50 intimate (colloided) mixture of carefully purified nitrocellulose and nitroglycerin, such a combination is not adequately stable for military use and would not perform well in rockets for numerous reasons. Nevertheless, the U.S. rocket propellant in World War II, "JPN" (Jet Propulsion/Navy), did consist mainly of nitrocellulose (51 percent) and nitroglycerin (43 percent) and it was based on long-used gun propellants. A fuel-type plasticizer, a new stabilizer, a wax, a blackening agent, and a potassium salt were used in minor proportions to improve processing, storability, and combustion properties respectively, but all the energy, or specific impulse (Isp), was derived from the nitrocellulose and nitroglycerin. In other words, all the necessary minor ingredients detracted from the basic ballistic performance or Isp. The JPN Ballistite served the U.S. for almost all military rocket needs during World War II in spite of its far-from-ideal internal ballistic properties. Double-base propellants generally require careful application of low flammability inhibitors if one desires to protect the internal motor walls from flame. Such materials represent an important field of study because satisfactory inhibitor materials must absorb a minimum of nitroglycerin, must be fully compatible with the propellant, and must not impose a severe increase in system cost. Ethyl cellulose and cellulose acetate are typical inhibitor materials for free-standing cartridge grains. Satisfactory case bonding liner-inhibitors generally involve a rubber substrate, impervious to nitroglycerin, and an intermediate film with excellent adhesive properties to propellant and liner. Other components of a rocket motor include the case and nozzle, fins, the igniter, mechanical devices to stabilize burning, and rubber seals to prevent leakage of gas or flow of hot gases between inhibitor and motor. COMPOSITE PROPELLANTS A feature of ammonium perchlorate, rubber-based, composite propellants is a natural tendency toward low pressure. True plateau burning is seldom achieved in high performance composite systems, but slopes ("n") of 0.3 or 0.4 are very common and are adequate to ensure operability over a wide range of temperatures. A twenty-year, multimillion-dollar effort on binders for ammonium perchlorate oxidizer, aluminum fuel, and energetic gas producers, such as nitroguanidine and HMX, has produced an array of composite propellants that
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